One of the conventionally known methods for manufacturing a graphite refractory article comprises: mixing and kneading a carbonaceous raw material such as a petroleum coke and an anthracite, and a binder such as coal tar and coal pitch, to prepare a wet or plastic mixture; applying press forming or extrusion forming to said mixture to obtain a formed body having a prescribed shape; and, applying firing at a high temperature of at least 2,000.degree. C. or electrical resistance heating at a high temperature of at least 2,000.degree. C. to said formed body, to convert said carbonaceous raw material into graphite (hereinafter referred to as the "prior art (1)").
However, the prior art (1), which requires a long period of time and a huge amount of heat energy for converting the carbonaceous raw material into graphite, is very low in efficiency.
Another method for manufacturing a graphite refractory article is known, which comprises: mixing and kneading from about 83 to about 87 wt.% natural flake graphite as the carbonaceous raw material and a binder such as coal tar and coal pitch to prepare a wet or plastic mixture; applying press forming or extrusion forming to said mixture to obtain a formed body having a prescribed shape; and, baking or firing said formed body (hereinafter referred to as the "prior art (2)").
In the prior art (2), which employs natural flake graphite as the carbonaceous raw material, it is not necessary to consume heat energy for converting the carbonaceous raw material into graphite as in the prior art (1). However, the structure of the graphite refractory article manufactured by the prior art (2), being composed of the flakes of natural flake graphite non-directionally overlapping one on top of the other at random, has a high porosity and is not dense. In addition, in the prior art (2), if the forming pressure to make a formed body is increased in an attempt to manufacture a graphite refractory article having a dense structure with a low porosity, the characteristics inherent in the natural flake graphite such as the flat shape and slipperiness cause such defects as laminations and cracks in the formed body. Because of the forming pressure thus limited of the formed body in the prior art (2), it is difficult to manufacture a graphite refractory article having a dense structure with a low porosity.
Both in the prior arts (1) and (2), the binder such as coal tar and coal pitch is used in a large quantity for rendering a mixture wet or plastic, and this leads to volatilization of large quantities of volatile matter when baking or firing a formed body. As a result, the manufactured graphite refractory article has a high porosity, and therefore problems arise in service because of low oxidation resistance, thermal conductivity and erosion resistance against molten slag, molten pig iron and molten steel.
For the purpose of solving the above-mentioned problems associated with porosity, a method for improving porosity was proposed, which comrises: impregnating a baked or fired formed body with a pitch or a resin having a high carbon content to fill pores in the formed body with the pitch or the resin, and baking or firing again said formed body to convert the pitch or the resin in the pores into carbon or graphite. According to this method, the manufactured graphite refractory article has a lower porosity, whereas the complicated processes require a long period of time for manufacture of the graphite refractory article, thus leading to higher costs.
Under such circumstances, there is a demand for the development of a graphite refractory article having a dense structure with a low porosity, manufactured with the use of natural flake graphite as the carbonaceous raw material, but a graphite refractory article provided with such properties is not as yet developed.